JP Patent Publication 62-93565A discloses one of such roller type speed reducers. The roller type speed reducer disclosed in JP Patent Publication 62-93565A includes an input shaft and an output shaft that are arranged coaxial with each other with their ends facing each other. A housing covers the opposed ends of the input and output shafts, and supports an internal gear having curved internal teeth on the inner periphery. Two eccentric disks are mounted, axially spaced apart from each other, on the end portion of the input shaft so as to be rotatable inside the internal gear. A cage is provided at the end of the output shaft facing the input shaft so as to be disposed between the internal gear and rolling bearings press-fitted on the radially outer surfaces of the respective eccentric disks. Pockets are formed in two rows in the cage such that the pockets in each row are circumferentially equidistantly spaced apart from each other and radially face one of the eccentric disks. The pockets in each row are fewer in number than the internal teeth. Rollers are received in the respective pockets so as to engage the internal teeth of the internal gear one after another while rolling along the radially outer surfaces of the rolling bearings.
In this arrangement, when the input shaft of the speed reducer rotates once, each of the rollers circumferentially moves by a distance equal to the circumferential width of one internal tooth while kept in meshing engagement with the internal tooth, due to rotation of the eccentric disks, causing the output shaft to be rotated at a reduced speed.
In order to smoothly transmit rotation, JP Patent Publication 62-93565A proposes to determine the profile of each internal tooth of the internal gear of the speed reducer so as to coincide with the curve outside of any one roller that is parallel to the trajectory of the center of the roller, when the output shaft is rotated a distance equal to one pitch of the teeth of the internal gear by the rotation of the eccentric disks, whereby all of the rollers contact internal teeth.
Component parts forming the above-described roller type speed reducer, such as the internal gear, eccentric disks, rolling bearings and rollers, have manufacturing errors. Conventional speed reducers are manufactured by simply assembling together these component parts, which have manufacturing errors, so that roller gaps tend to vary from one speed reducer to another, and thus their qualities also tend to vary.
One of the roller gaps is shown at 20 in FIG. 5, which shows a roller reduction mechanism of a roller type speed reducer. As shown in FIG. 5, the roller gaps 20 are defined between rollers 19 arranged on the side of a rolling bearing 11 fitted on the outer periphery of an eccentric disk where the eccentric disk is displaced and tooth bottoms 4a of internal teeth 4 formed on the inner periphery of an internal gear 3.
Since conventional speed reducers are assembled without controlling the roller gaps 20, it is difficult to set the size of the roller gaps 20 within an optimum range. Thus, the roller gaps 20 may be too large in some speed reducers. If the roller gaps 20 are too large, when a roller 19 disengages from one internal tooth 4 of the internal gear and then engages an adjacent internal tooth 4, the roller 19 tends to collide against the tooth bottom 4a of the internal tooth 4, thus producing vibration.
The size δ1 of the roller gaps 20 are given by:δ1=(A−B)−C  Formula (1)whereA is the root radius of the internal gear 3;B is the radius of the circumcircle of the rolling bearing 11, of which the center lies on the center axis of the input shaft; andC is the outer diameter of the rollers 19.
The present inventors examined influences of the roller gaps 20, which are defined between the rollers 19 and the tooth bottoms 4a of the internal gear 3, on the speed reducer, and discovered the following:
Efficiency: If some roller gaps are too large and other gaps are too small, the speed reducer cannot rotate at a constant speed. Also, torque loss is large where roller gaps are small, which reduces efficiency.
Life: Excessive surface pressures are generated between the contact portions of a roller and the radially inner surface of the internal gear or between the roller and the outer race of the rolling bearing when the roller passes through an excessively small roller gap, which could result in premature peeling.
Vibration: Behaviors of the rollers become unstable when they pass through excessively small roller gaps, producing vibration.
FIG. 6 shows the results of characteristic evaluation of the roller gaps 20 in terms of input characteristic values. FIG. 6 clearly indicates that it is important that the roller gaps 20 be within an optimum range.